Electronic tuning means



May 28, 1957 B. D. KUMPF ER ELECTRONIC TUNING MEANS Original Filed May5, 1950 Il l//l/ /l/ FUU UUUU' UUU I //I//I II l5 i I FIG. 2

United States Patent ELECTRONIC TUNING MEANS Beverly D. Kumpfer, SpringLake Heights, N. J., assigner to the United States of America asrepresented by the Secretary of the Army 1950, Serial No. 159,800,

Original application May 3 dated June 14, 195s. Di-

now Patent No. 2,710,919,

vided and this application September 23, 1952, Serial The inventiondescribed herein may Abe manufactured Iand used by or for the Governmentfor governmental purposes, without the payment of any royalty thereon.

This application is a division of application Serial No. 159,800 iiledMay 3, 1950, now Patent No. 2,710,919.

The inventio-n relates to a device for electronically tuning resonantstructures and more particularly to microwave generators employing waveguides `and cavity resonators whose frequency characteristics can bevaried by electronic means. While the invention is subject to a widerange of applications, it is especially suited for use with, microwavegenerators -such as magnetrons and klystron tubes whose outputwavelength depends upon the dimensions of an external tuned circuit.

In conventional microwave generator circuits, the output -frequency iscontrolled primarily =by the resonant frequency of the cavity resonator.When klystrons or magnetrons are employed, the operating frequency isprimarily a function of the cavity resonator dimensions which may bechanged mechanically or electronically. Mechanical tuning is generallyaccomplished Iby employing plungers to change the physical dimensions ofthe cavity resonator while electronic tuning is yachieved by injectingelectrons into the resonant system in order to change the electricalcharacteristics thereof. Heretofore, electronic tuning for bothklystrons and magnetrons has been accomplished by injecting an electronbeam of variable intensity into a region of high R.F. electric fieldswithin the resonator. For magnetron generators, the variable intensityof the eletctron beam is analogous to a variabledielectric constant inthe cavity resonator, 'hence a variable resonant frequency of theoscillator.

When magnetrons are employed, the tuning range for eicient operation islimited by the fact that suiiciently high beam current necessary forwide frequency deviations are difficult to achieve, and moreover, inview of the high R.F. fields present in the magnetron cavity, no controlof the high density beam within the cavity would be possible. In tuningreflex klystrons, it is well known that for efficient `operation thetuning range can be varied electronically only between narrow limits,the frequency deviations varying only slightly on either side of thefundamental resonator frequency.

Accordingly, it is an object of this invention to provide an electricdischarge device which operates to electronically tune a resonantstructure over a Wide range of frequencies.

It is still another object to provide a resonator whose frequency iselectronically varied by adjusting the position of an electron spacecurrent within said resonator.

In accordance with my inventiona microwave generating apparatuscomprises a resonant circuit and means v for producing and controllingan electron space current, or space charge, within said resonant circuitto effectively tune the output of the microwave generator through a widerange of frequencies.

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For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription taken in connection with the accompanying drawings in which:

Fig. l illustrates schematically and in longitudinal section oneembodiment constructed in accordance with the principles of the presentinvention;

Fig. 2 illustrates schematically and in longitudinal section a preferredembodiment of my invention;

Fig. 3 is a transverse section through the resonant structure shown inFig. 2, the plane of section being indicated by the line 6-6 of Fig. 2;

Fig. 4 illustrates an embodiment of my invention ernpioying resonantcavities of the pill-box type; and

Fig. 5 is a perspective view of the grid and cathode structure employedin Fig. 4.

Similar characters of reference are used lin all of the above figures toindicate corresponding parts.

Referring now to Fig. 1 there is shown `a lbroad-band microwave tuningdevice comprising an evacuated wave guide 10, a high frequencygenerating source 15 enclosed therein, a cathode 16 centrally positionedwithin a portion of wave guide i0, a variable spaced grid 17 surroundingcathode 16 for the entire length thereof, `a potential source 23 whichis applied between wave guide 10 and cathode la, and grid voltagemodulating source 24. The showing of Fig. l is a schematic in that theconstruction of the high frequency generating source, which may comprisea suitable magnetron oscillator, the `arrangement lfor applyingoperating potentials thereto, and means for its support within waveguide 10 `are not shown. Such details are well known 4and do notconstitute Iany part of my invention.

As shown in Fig. l, evacuated rectangular wave guide 10 is closed atboth ends to form ya resonant structure. The rectangular' cross-sectionof wave guide 10 jhas a major dimension somewhat larger than one-half ofthe maximum Wavelength of the energy corresponding to the lower limit ofthe frequency range `through which it is desired to operate, and a minordimension much smaller than one-half of said wavelength. Generatingsource 15 may be spaced from closed end 1l by dimensions correspendingto odd-quarter multiples of the 4wavelength in the guide of thefrequency which is mid-Way Abetween the' upper and lower frequencylimits of the desired operating range. The output of generating source15 is so polarized that a transverse electric wave, or TE mode, isgenerated within wave guide 10. Thus, electric lines of force extendbetween the two wide walls 13 and 14 of the wave guide. Ribbon-likecathode 16 is centrally positioned along the longitudinal axis of waveguide 10 be'- ween closedA end 12 and generating source 15 `and iscoated with an electron emissive material only on the twoV surfacesfacing said wide walls. It is supported in posi-v tion by lead 18 whichextends through `an aperture in closed end l2 and through glass seal 19.lf further support is necessary a mica spacer (not shown) may beemployed in the conventional manner at the generator end. The length ofcathode 16 Within wave guide 10 is determined by the desired maximumoperating frequency asexplained below. Grid 17 surrounds cathode 16 forthe entire length thereof and the spacing between successive grid turnsprogressively decreases at Ia constant rate so that the grid wiressurrounding cathode 16 at the generator end will be closer together thanthe grid wires lat the other end of said` cathode. Thus, as shown inFig. 1 the pitch between grid wires are progressively decreased as theyapproach generating source 15. Wave guide 10 is maintained at a positivepotential with respect to cathode 1 6 by D. C. voltage source 23 whichis applied between saidV cathode and theouter surface of the wave guide.f t

In the operation of the device illustrated in Fig. l, an electron cloud,or space charge, is created in the region between the coated surfaces ofcathode 16 and wide walls 13 and 14 due to the electron space currentbetween said cathode and wave guide 1t). lt will be shown in thefollowing, that variations in the length of the space current chargealong the cathode and incidentally along the longitudinal axis of thewave guide 1t), may be used to modify the frequency of resonance of thestructure, hence vary the output frequency of generator 15.

As has been previously explained, transverse electric lines of force aregenerated in wave guide 1G by high frequency generating source 15' and,since the space charge is parallel to the lines of electrostatic force,said space charge forms a transverse electronic short between wide walls13 and 14. The spacing between the innermost position of the electronicshort and generator' 15 determines the operating resonant frequency.This is equivaient to the complete elimination of the section of waveguide 1t) between closed end 12 and the innermost position of thetransverse electronic short end will have results similar to decreasingthe length of the wave guide. Reducing the length of the wave guide willdecrease the resonant wavelength of the structure and thus tend toincrease the frequency output from source 15, with which it isassociated. Thus, with the space charge distributed along thelongitudinal axis of the wave guide for the entire length of cathode 16,the resonant frequency will be at a maximum. Accordingly, the length ofcathode 16 within wave guide 10 is determined by the upper limit, ormaximum frequency, of the desired frequency range.

The innermost position of the transverse electronic short across thewave guide may be varied by applying to grid 17 a source 24 of variablevoltage with which it is desired to modulate the output of generatingsource 15. Due to the graduated spacing of the grid Wires, variations inthe potential supplied by source 24 will produce corresponding changesin the position of the transverse electronic short, or space charge,within wave guide 10. For relatively small negative potentials, electronspace current will be cut-off from only that portion of the cathodesurrounded by the closer grid spacing. Similarly, as the applied gridvoltage is increased negatively, electron space current will be cut offfrom a greater portion of the cathode. The ultimate result will be tomodify the electrical length of wave guide and tuning of the resonantstructure, and hence to change the frequency of the voltage developed bygenerator source in accordance with the signal pattern applied to grid17. Thus, for example, if the grid voltage is varied linearly byapplying a negative sawtooth voltage thereto as shown, the electronspace current, hence the space charge, is periodically moved linearlyalong the longitudinal axis of wave guide 10. Moving the transversespace charge toward closed end 12 will decrease the resonant frequency,the minimum frequency limit being achieved when the space current is cutorf for substantially the whole length of the cathode.

While in the particular embodiment there has been described arectangular wave guide resonant structure, it will be understood thatother resonant structures such as a coaxial line may be used. It shouldbe further understood that the particular spacing of the grid wireswhich has been described in connection with Fig. 1 is not essential tothe purposes of the invention. For example, the grid spacing may varyexponentially if it is so desired.

In the arrangement shown in Fig. 2, a double ridged wave guide 30constitutes the resonant structure within which cathode 16 islongitudinally positioned between ridges 31 and 32. Cathode 16 is coatedwith an electron emissive material on only those surfaces facing innerridge surfaces 31 and 32 so that the space charge emitted therefrom isconcentrated within the ridged area. Thus, the electronic short isconcentrated in the region of greatest density of electric lines offorce. The operation of this device is identical to that explained inconnection with Fig. l.

A still further application of the invention is represented in Fig. 4which shows its use in a pill-box cavity resonator to adjust theresonant wavelength thereof. As shown in Fig. 4, high frequencygenerating source 15 is axially positioned within pill-box resonator 40.The construction of the high frequency generating source and means forits support within resonator 4t) and the arrangement for applyingpotentials thereto are not shown, since such details are well known inthe art.

Annular cathode 44 is centrally positioned within the resonator and theouter periphery thereof is radially spaced from lateral wall 41. Cathode44 is coated with an electron emissive material only on the two surfacesfacing top and bottom resonator plates 42 and 43, respectively.Surrounding cathode 44 is a toroidally wound grid 45 so that the spacingbetween adjacent grid wires increases from the inner periphery ofannular cathode 44 to the outer periphery thereof. Thus, adjacent gridwires are closer at the inner periphery of cathode 45 than at the outerperiphery of said cathode. Both cathode 44 and toroidally wound grid 45are supported in position by leads such as 46 and 47 which extendthrough lateral Wall 41 to glass seals 48 and 49, respectively. A D. C.voltage 23 is applied between cathode 44 and resonator 4!) to create aspace charge between the coated surfaces of the cathode and the innersurfaces of top and bottom plates 42 and 43. This space chargeconstitutes a cylindrical electronic short between said top and bottomplates. The radial position of the cylindrical electronic short may bevaried by applying to grid 45 a source 24 of variable voltage with whichit is desired to modulate the output of generator source 15. Due to thevaried spacing of the grid wires at the inner periphery and the outerperiphery of the annular cathode, variations in the potential suppliedby source 24 will produce corresponding changes in the radial positionof the cylindrical electronic short within resonator 4t). For relativelysmall negative potentials electron space current will be cut olf Vfromonly that portion of the annular cathode surrounded by the closer gridspacing. Similarly, as the applied grid voltage is increased negatively,electron space current, hence the space charge, will be cut off from agreater portion of the cathode. As previously explained, the tuning ofthe resonant structure and, hence the frequency of the voltage developedby source 15 will be varied in accordance with the signal rpatternapplied to grid 45.

While the invention has been described by reference to particularembodiments thereof, it will be understood that numerous modificationsmay be made therein by those skilled in the art without departing fromthe invention, and it is, therefore, aimed in the appended claims tocover all such modifications as fall within the true spirit and scope ofthe invention,

What is claimed is:

l. In combination, an evacuated cylindrical resonator, a source of highfrequency electromagnetic waves having a transverse electric fieldaxially positioned within said resonator, the output frequency of saidsource being a function of the resonant wavelength of said resonator, anannular cathode centrally positioned within said resonator and radiallyspaced from the lateral wall thereof and which is electron emissive onlyon the two surfaces facing the top and bottom plates of said resonator,means for producing an electron space charge coplanar with saidtransverse electric field whereby said ield is short circuited, and acontrol electrode surrounding said cathode, said control electrodehaving a cutoff characteristic which varies from the inner periphery ofsaid cathode to the outer periphery of said cathode whereby theeffective circumferential space charge may be varied to modify theresonant wavelength of said cylindrical resonator.

2. The combination in accordance with claim 1 wherein said controlelectrode is a grid toroidally wound around said cathode.

3. The combination in accordance with claim 2 wherein the grid windingturns are closer at the inner periphery of said cathode than at theouter periphery thereof.

References Cited in the ile of this patent UNITED STATES PATENTS2,241,976 Blewett et a1. May 13, 1941 6 Schmidt Dec. 31, 1946 Fiske Feb.18, 1947 Spencer Apr. 26, 1949 Spencer July 26, 1949 Sproull Mar. 21,1950 Derby Ian. 16, 1951

